1. Field of the Invention
The invention relates to a magnetic random access memory (MRAM), and in particular a magnetic tunnel junction (MTJ) device for a MRAM.
2. Description of the Related Art
A magnetic random access memory (MRAM) typically comprises a combined magnetic multi-layer structure and a transistor, and provides better radiation resistance than conventional semiconductor materials. A magnetic random access memory (MRAM) is a non-volatile random access memory capable of retaining data after power-off, and is thus suitable for application in information, communication and computer products.
Data in a magnetic random access memory (MRAM) is recorded by changing magnetoresistance characteristics thereof, providing advantages such as low power consumption, non-volatile memory behaviors and unlimited writing and reading. Memory units in the MRAM are typically formed in a stacked structure including an antiferromagnetic (AFM) layer of antiferromagnetic materials, a pinned layer of magnetic materials, a barrier layer, and a free layer of magnetic materials. While data is written, a memory unit can be selected by providing induced magnetic fields at two separate conductive lines, for example a bit line and a data line, to thereby change direction of the magnetization of a free layer and the magnetoresistance of a selected memory unit. Thus, while data is read, a bit status of the stored data can be obtained by distinguishing the magnetoresistance (MR) thereof. In general, whether a memory status of a memory unit is under “1” or “0” can be determined by distinguishing whether the magnetization of the free layer and the pinned layer adjacent to the barrier layer are in parallel or antiparallel. The memory status stored can be kept forever and is not changed until a magnetic field is applied thereto.
Nevertheless, when designing high density MRAM, memory unit size in thereof must be reduced to increase the number of units formed in a predetermined area. With the trend of memory unit size reduction, a switching field applied to the memory units is thus increased, thereby increasing a current applied on the conductive lines. In addition, switching uniformity of a free layer of all the memory units in a MRAM must be considered to increase a writing window thereof. Thus, a memory unit structure must be improved to reduce switching fields, current applied to the conductive lines, and narrow variation of switching field.
Thus, memory units having great MR ratio, faster reading speed, and uniform switching behavior, and wider writing window are needed in high density MRAM applications to thereby simultaneously reduce switching fields and currents applied to the conductive lines and to unite the switching behaviors of the memory units therein.